Among surface light emitters, an organic EL (electroluminescence) light emitting device is expected to be used in a flat panel display or in a new generation lighting device that is used in place of a fluorescent bulb and the like.
The organic EL light emitting device has various structures including a simple structure in which an organic thin film, which is a light emitting layer, is simply sandwiched by two films and a multi layered structure. As the multi layered structure, a structure in which a hole transporting layer, a light emitting layer, an electron transporting layer, and a cathode are layered on an anode provided on a glass substrate can be exemplified. The layers that are interposed between the anode and the cathode are all composed of organic thin films and a thickness of each organic thin film is as thin as several dozen nm.
The organic EL light emitting device is a layered body of thin films and a total reflection angle of light between the thin films is determined by difference of refractive index between materials of the thin films. Currently, approximately 80% of light generated in the light emitting layer is confined in the organic EL light emitting device and cannot be extracted therefrom. More specifically, given the refractive index of the glass substrate being 1.5 and the refractive index of an air layer being 1.0, a critical angle θc is 41.8°, and light with an incidence angle smaller than the critical angle θc is output to the air layer while light with an incidence angle greater than the critical angle θc is totally reflected and confined in the glass substrate. For this reason, extraction of the light confined inside the glass substrate on a surface of the organic EL light emitting device to the outside of the glass substrate, in other words improvement of light extraction efficiency and normal brightness, is desired.
Meanwhile, for the organic EL light emitting device that emits light isotropically, in addition to improvement of light extraction efficiency and normal brightness, an output light wavelength from the organic EL light emitting device with low dependence on an output angle is desired. That is, upon output of light from the light emitting layer through the glass substrate, it is desired that a difference in output angle between wavelengths is small, in other words, it is desired that wavelength dependence is as small as possible with regard to distribution of output light from the glass substrate.
In addition, providing an optical film on an output surface of the organic EL light emitting device is known as a method for solving the above described problem; however, since the optical film is provided in an outermost layer, flaw resistance is desired.
Patent Document 1 proposes an optical film having a microlens covered by an outer layer composed of a deposited material with a low refractive index, in order to improve brightness of a surface light emitter. Patent Document 2 proposes an optical film having a lens portion containing fine particles in order to maintain uniformity of brightness of a surface light emitter.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2011-123204
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2009-25774